Vad är First Input Delay?
Snabbdefinition
First Input Delay (FID) mäter tiden från en användares första interaktion med en sida tills webbläsaren kan börja bearbeta den interaktionen.
First Input Delay captures the delay users experience when they try to interact with a page that's still loading. If a user clicks a button but the browser is busy parsing JavaScript, there's a gap between the click and the response. That gap is FID.
FID is important because it measures real user experience, not lab data. It can only be measured in the field because it depends on actual user behavior. Google's Web Vitals program considers FID good if it's under 100ms, needs improvement between 100-300ms, and poor above 300ms.
The primary cause of high FID is heavy JavaScript execution during page load. When the browser's main thread is busy parsing and executing JavaScript, it can't respond to user interactions. Third-party scripts (analytics, ads, chat widgets) are common culprits.
Note: Google has replaced FID with Interaction to Next Paint (INP) as a Core Web Vital, which measures all interactions throughout the page lifecycle, not just the first one. However, FID remains a useful concept for understanding initial page interactivity.
Reducing FID involves minimizing and deferring JavaScript, breaking up long tasks, using web workers for heavy computation, and implementing code splitting to reduce the amount of JavaScript that runs during initial page load.
Varför det spelar roll
Users notice delays as short as 100ms. When they click a button and nothing happens, they click again, or they leave. High FID creates a perception that your website is broken or slow, even if it eventually loads correctly.
Since FID (and its successor INP) is a Core Web Vital, it directly affects Google search rankings. Sites with poor interactivity metrics may rank lower than faster competitors.
Verkliga exempel
A news website had 500ms FID because their ad scripts were monopolizing the main thread during load — deferring these scripts reduced FID to 50ms
An e-commerce site's product page had 300ms FID from a heavy JavaScript bundle — code splitting and lazy loading reduced it to under 80ms
A marketing site removed 3 third-party chat and analytics scripts and improved FID from 250ms to 30ms, with minimal impact on functionality
A web app moved their data processing logic to a Web Worker, freeing up the main thread and reducing FID from 400ms to 60ms on mobile devices
Relaterade termer
Core Web Vitals
Core Web Vitals är tre specifika mätvärden från Google som mäter verkliga användarupplevelser på webbplatser: laddningsprestanda (LCP), interaktivitet (INP) och visuell stabilitet (CLS).
Page Speed
Sidhastighet är ett mått på hur snabbt en webbsida laddar och blir interaktiv, vilket direkt påverkar användarupplevelsen och sökmotorrankningar.
Code Splitting
Code splitting är en optimeringsteknik som delar upp en applikations JavaScript-kod i mindre delar som kan laddas vid behov, vilket minskar den initiala paketstorlekan.
LCP (Largest Contentful Paint)
LCP mäter den tid det tar för det största synliga innehållselementet på en sida, som en hero-bild eller rubrik, att renderas fullständigt.
Behöver du hjälp med first input delay?
Vårt team kan hjälpa dig att tillämpa detta koncept i praktiken. Boka en kostnadsfri konsultation för att diskutera ditt projekt.